Latex paints are used for a variety of applications including interior and exterior, and flat, semi-gloss and gloss applications. However, paints and aqueous latex dispersions, particularly low VOC paints and latex dispersions, suffer from a lack of freeze-thaw stability. This is particularly a problem during transportation and storage.
Latex freeze-thaw (sometimes herein referred to as “F/T”) stability, including the freezing-thawing process, destabilization mechanism, and polymer structures, have been extensively studied since 1950. Blackley, D. C., Polymer Lattices-Science and Technology, 2nd Ed., Vol. 1, Chapman & Hall, 1997, gives a comprehensive review of colloidal destabilization of latexes by freezing. The freezing process starts with the decrease of temperature which leads to the formation of ice crystals. The ice crystal structures progressively increase the latex particle concentration in the unfrozen water. Eventually latex particles are forced into contact with each other at the pressure of growing ice crystal structures, resulting in particle aggregation or interparticle coalescence. To make a stable latex dispersion in aqueous medium or latex paints with freeze-thaw stability, various approaches have been employed. The addition of antifreeze agents, e.g. glycol derivatives, has been applied to latex paint to achieve freeze-thaw stability. Thus, latex paints include anti-freeze agents to allow the paints to be used even after they have been subjected to freezing conditions. Exemplary anti-freeze agents include ethylene glycol, diethylene glycol and propylene glycol.
See, Bosen, S. F., Bowles, W. A., Ford, E. A., and Person, B. D., “Antifreezes,” Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A3, VCH Verlag, pages 23-32, 1985. However, a low or no VOC requirement means the glycol level that can be used has to be reduced or eliminated. Aslamazova, T. R., Colloid Journal, Vol. 61, No. 3, 1999, pp. 268-273, studied the freeze resistance of acrylate latexes and revealed the role of electrostatic contribution to the potential energy of latex particle interactions. Using electrostatic effects on colloidal surface, the interactions of Coulombic repulsion between the charged latex particles lead to higher potential energy. The electrical effects stabilize the latex particles in the freezing and thawing process.
Rajeev Farwaha et. al. (U.S. Pat. No. 5,399,617) discloses the copolymerizable amphoteric surfactants and discloses latex copolymers comprising the copolymerizable amphoteric surfactants impart freeze-thaw stability to the latex paints.
Cheng-Le Zhao et. al. (U.S. Pat. No. 6,933,415 B2) discloses latex polymers including polymerizable alkoxylated surfactants and discloses the low VOC aqueous coatings have excellent freeze-thaw stability.
Rajeev Farwaha et. al. (U.S. Pat. No. 5,610,225) discloses incorporating a monomer with long polyethylene glycol structures to achieve stable freeze-thaw latex.
Masayoshi Okubo et. al. (U.S. Pat. No. 6,410,655 B2) discloses freeze-thaw stability of latex polymers including ethylenic unsaturated monomers.
The additives used as anti-freeze agents are effective for their purposes but are becoming more and more undesirable because they are volatile organic compounds (VOC's). After application of the latex paint to a substrate, the VOC's slowly evaporate into the surroundings.
With strict environmental legislation requiring the reduction of the amount of Volatile Organic Compounds (VOC) in coatings, it is desirable to have paint formulations without or with substantially reduced VOC content, which would include coalescing agents and freeze-thaw agents, among others. Latex binder manufactures are thus forced to develop low VOC binders to meet the requirements of paints and coatings industry. However, low VOC coatings and paints must meet or exceed coating performance standards set in the industry.
In traditional latex binders for architectural coatings, the glass transition temperature is between above 10° C. to about 40° C. However, such architectural coatings often need and contain coalescent agents to soften such latex binders (i.e., soften the latex binders having relatively Tg in the range of above 10° C. to about 40° C.) or anti-freeze agents; both of which are typically high VOC solvents. Thus, these traditional architectural coatings with higher Tg latex binders cannot be formulated to be low VOC without solvents.
For the low VOC application (i.e., low Tg) binders, the average Tg is close to or below 0° C. However, the latex binder with low Tg causes grit when subjected to freeze/thaw cycles as well as exposure to mechanical shear. The resulting coating films are softer and tackier, even after fully dried, and are susceptible to blocking and dirt pick-up effects. Also, such low Tg latex binders and resulting latex paints are not stable, and gel in a cold environmental storage or transportation process. Freeze-Thaw stability of low Tg latex binders and low VOC paints is critically important for transportation, storage, and practical applications. Thus, there is a need to develop latex paints and latex particle dispersions using emulsion polymer technology which meet zero or low VOC requirement and at the same provide excellent mechanical and film performance without sacrificing the freeze-thaw stability of those paints.